There is an urgent need to improve the performance of diode laser systems. The overall commercial laser market has expanded from $4B in 1998 to about $6B in 2002. The diode laser share of this market has remained stagnant at about 33% during the past four years, due in part to the high complexity of current pivot mirrors that control the laser cavities. The associated fabrication costs limit several key applications of these lasers. Despite these limitations, financial investment in the diode share of the laser market remains surprisingly strong. Private investment in novel optical components has only declined from $1.8B to $1.1B during the past year. During this same period investment in new ventures in the U.S. has suffered a much more drastic decline, from a high of $134B in 2000 to less than $30B after 2002, due in large part to an ongoing recession associated with the violent events beginning in September 2001.
This relative vote of confidence of investors in the overall laser market is due in large part due to the large increases in defense applications expected in coming years. A second growth factor is the surprisingly strong growth in sales of optical systems for fluorescence sensors recently (currently doubling every 3 to 5 years). This industry can contribute to the market growth of visual diode lasers, since they are often the least expensive and most compact laser technologies to be used as light sources in these fluorescence systems. These light sources are currently used to excite all the recently developed blue, green and red dyes for detection of many novel types of fluorescent polymers. The rapid growth of the number of fluorescent agents has dramatically increased the accuracy of biochemical detection for an increasingly wide variety of proteins, enzymes, antibodies and nucleic acids. There is an expanding need for these optical products in the fields of agribusiness, food processing, environmental sensors, laboratory work and process efficiency monitors. The pharmaceutical, biotechnology and medical industries are becoming increasingly dependent on a wide variety of inexpensive optical testing and monitoring of many critical processes used in many areas, from food safety to drug production.
Current diode laser systems do have disadvantages relative to gas and other solid-state lasers. If these lasers are not controlled by external cavities, they will unpredictably change their output wavelength. These are called internal cavity mode hops. The internal Fabry-Perot cavities that generate the laser light are capable of operating in a number of possible modes (orders). These modes are made possible by the different thickness of the semiconductor quantum wells that form this cavity. Each laser mode will radiate at a specific narrow band of wavelengths. Shifts in these modes occur randomly and are also induced by temperature or voltage drifts, resulting in changes in the spectrum of the output beam. Several external cavity designs can greatly reduce the spectral variations of the laser by redirecting only a narrow portion of the potential spectrum toward the internal cavity. Diode lasers are one of the most compact and least expensive laser technologies, so their control systems should also be correspondingly inexpensive and compact.
Scoby and Zhang (2000) disclose a laser cavity with a monolithic prism assembly, but such a prism design does not disperse light. Sacher (1999) discloses a tuning arrangement for a semiconductor diode laser using a rectangular prism refraction grid, but there are large reflective losses from the surfaces of these prisms and the dispersion is lower than for the Brewster angle wedges. There are many other examples of earlier patents using several types of prisms besides those referenced here, but all lack certain capabilities. For example, they all employ a classical prism and are thus expensive to mount and adjust. (Specific semiconductor prisms, e.g. those made of ZnSe, have been successfully used for spectroscopy and are now covered by the patent authored by Fay (1997). However, there is a need to improve cavities for laser diodes.)
Two broad examples of laser cavity control systems that do not use prisms or wedges are: the Littman Metcalf (grating and retro-reflector) and the Pound, Drever and Hall (Fabry-Perot and phase modulator) systems. Present Littman Metcalf designs require very complex pivot motion control. They also have lower spectral resolution and efficiency than the Brewster angle wedge designs claimed in this provisional patent. Past inventions have attempted to overcome the basic design deficiencies of these external cavities by increasing both their size and the cost. The laser diodes themselves can be purchased from a number of manufacturers for between $1000 and $10,000, their cost depends on the power requirements demanded by the customer. The fabrication cost of high quality external cavities is now higher than the laser diode that is to be controlled. These cavities can be purchased at New Focus for between $15,000 and $30,000 for Littman Metcalf optics and over $50,000 for the Pound, Drever and Hall optics. The least expensive biochemical fluorescent systems cost just a little over $50,000, and these do not employ stabilized cavity lasers. An external diode laser cavity with lower fabrication costs will both reduce the cost of fluorescent devices and improve sensitivity by factors of up to ten.
Several companies manufacture a number of different kinds of external laser cavities that use gratings. Two recent examples of improved external cavity patents commercially available from New Focus are: Moore, et al. (1999) and Lueke (1994), a modification of the pivot controls for the Littman Metcalf system. The active optical element of the Littman Metcalf control system is the grating, the number of grooves limit the resolution of this system to about 10,000/cm of grating surface. The diffracted light from this grating is simply retro-reflected back toward the grating at a particular angle by a mirror or wedge that must both rotate and translate on a pivot assembly. The sine of the chosen diffraction angle to be pivoted is directly related to the wavelength through the groove spacing.
The Littman Metcalf grating method uses precision optical surface grooves (quarter wavelength or less) to diffract the laser light of a given wavenumber at a particular cavity angle required for a stabilized feedback. The translation motion of the reflector is set precisely (sub micron accuracy) to give a cavity whose optical path is either an integral or half integral of the desired wavelength. The grating has an angular dispersion parameter that is proportional to the tangent of the blaze angle of the grating. This dispersion parameter limits the accuracy and sensitivity of the wavelength tuning. The resolution of the grating is proportional to the product of its width, the laser wavenumber and the sine of the incident angle plus the sine of the diffraction angle.
The three major disadvantages of operating the grating at high angles of incidence to achieve high resolution are as follows. (1) The scattered light does increase with both incident and diffraction angle. Such scattered light not only results in beam energy loss, but it contaminates the spectral and spatial purity of the laser beam controlled by this type of cavity. (2) The ideal grating efficiency decreases with the increasing sine of the blaze angle and with the tangent of the diffraction angle. This particular light loss factor is due to the geometric shadow created by the tilt of the grooves (blaze) relative to the incident angle of the laser beam. (3) The reflection and absorption loss coefficients at the grating surface also generally increase with angle.
Laser beam losses at the grating contribute to laser mode hop instability. In the Littman Metcalf cavities this instability occurs near the limits of the cavity wavelength tuning range, where internal cavity light production also declines. As discussed above, the grating scatters light from only a fraction of the surface that depends on the grating blaze angle and the incident light path angle. The absorption losses at the surface of the grating depend on the coating quality. All of the above factors reduce grating performance efficiency and restrict the tuning range for diode laser systems. Littman Metcalf tuning ranges are currently as large as 10 nm at visible wavelengths (fluorescent biotechnology band applications) and up to 70 nm in the near infrared (telecommunications band applications). The widths of the latter bands are standardized; so their number depends on the tuning range.
The active element of the Pound, Drever and Hall, or PDH, cavity uses a Fabry-Perot interferometer. Day and Marsland (1993) of New Focus have one of the more recent patents on the Pound, Drever and Hall system that is currently in commercial production. The patent office now documents many other types of external cavities that employ Fabry-Perot filters. These cavities overcome many of the light loss limitations of the Littman Metcalf system described above. A Fabry-Perot with an etalon separation of 3 cm can have a wavelength resolution that can be 1000 times higher than a grating, depending on the etalon flatness and coating reflectivity. The spectral purity of these cavities can also be up to 20 times higher than a grating and their peak wavelength efficiencies can be as high as 98%. Such cavities do have their own disadvantages because interferometers have multiple orders that overlap at the same optical angle. The particular PDH solution to the spectral overlap between orders is add a phase modulator and photo-detector offset by a beam splitter. These components both monitor part of the output beam and control the spacing of the etalons with piezoelectric means. This type of control system creates an integral or half-integral number of wavelengths in the cavity.
The PDH cavity control system is more expensive than Littman Metcalf and is one reason the PDH system has not yet become as popular a commercial solution as the grating cavities. Another reason is that both the phase modulator and the photo-detector with beam splitter both remove light from the beam, reducing cavity efficiency. These losses also contribute to mode hop instability at the limits of the tuning range just as occurs in the Littman Metcalf system. The Fabry-Perot component remains an excellent optic for achieving high purity and high spectral resolution in an external cavity, often more than 100,000, or ten times that of Littman Metcalf.
Wedges made of most optical materials have not been favored in most external cavity designs in the past, because the optical materials considered have factors of 10 or lower dispersion and resolution than gratings. Fay (1997) has patented several high dispersion semiconductor wedge materials for Raman spectroscopy.
There is a need for optical cavities that can accomplish the following: (1) simplification of the pivot motions that control the cavity spectral performance, (2) factors of from 2 to 4 higher dispersions than gratings, (3) a factor of 2 to 16 increase in light throughput, or angular field of view of the input and output fiber assemblies, (4) factors of from 20% to 3 higher spectral resolution, (5) a factor of 20% higher efficiency and (6) correspondingly lower levels of scattered light by factors of 10 or more. The last advantage significantly helps in improving laser diode performance at the edges of the tuning range. In addition, (7) business needs dictate that performance of the external cavity in many applications must not add significantly to fabrication costs.
The following is a list of references, each of which is incorporated herein by reference:    Day, T. and Marsland, R. 1993 “Electro Optical Light Modulator Driven by a Resonant Electrical Circuit”, U.S. Pat. No. 5,189,547, Assignee: New Focus, Mountain, View, Calif. (the Pound, Drever and Hall Fabry-Perot cavity patent).    Fay, T. 1997 “Compact Laser Diode Monitor Using Defined Laser Momentum Vectors to Cause Emission of a Coherent Photon in A Selected Direction”, U.S. Pat. No. 5,617,206, Assignee: PHI Applied Physical Sciences, Mission Viejo, CA 1 Apr. 1997.    Luecke, F. 1994 “Tuning System of an External Cavity Diode Laser”, U.S. Pat. No. 5,319,668, Assignee: New Focus, Sunnyvale, Calif.    Moore, B., Arnone, D., MacDonald, R. and Lueke, F. 1999 “External Cavity Laser Pivot Design” U.S. Pat. No. 5,995,521, Assignee: New Focus, Santa Clara, Calif.    Sacher, J. 1999 “Tuning Arrangement for a Semiconductor Diode Laser with an External Cavity Resonator”, U.S. Pat. No. 5,867,512, Marburg, D. E.    Scobey, M. and Zhang, X. 2000 “External Cavity with a Monolithic Prism Assembly”, U.S. Pat. No. 6,115,401, Assignee: Corning, OCA, Corning, N.Y.